A large number of hydrocarbon charges used in the oil industry, such as certain crude oils, straight-run residues or vacuum residues, shale or bituminous sand oils, or products from coal liquefaction, are characterized by a high content of asphaltenes and of such metals as nickel and vanadium; accordingly, they cannot be directly subjected to the conventional refining treatment such as catalytic cracking, hydrocracking or hydrodesulfurization, for example.
As a matter of fact, the metals and an asphaltenic carbon fraction remain fixed onto the catalyst, obstructing the pores, destroying the activity of the active centers and generating pressure drops. As a result, the catalyst charge must be renewed at a more frequent rate as the metal and asphaltene contents are higher. It is thus convenient to divide the asphaltenic charge into two fractions: a fraction essentially formed of asphaltenes and containing the major part of the metals and a complementary fraction formed of deasphalted oil. The more currently used separation technique, disclosed in the prior art, is the precipitation of asphaltenes by addition to the asphaltenic oil of suitable amounts of light hydrocarbons under convenient conditions. For the purpose of selectively precipitating asphaltenes, the selected solvents consist of light hydrocarbons, paraffinic or olefinic, preferably containing 3 to 8 carbon atoms, used either pure or preferably as mixture.
This deasphalting operation must be achieved as selectively as possible in order to obtain a maximum yield of deasphalted oil. The yield of deasphalted oil obiviously depends on the asphaltene content of the treated charge and on the nature of the asphaltenes. The selectivity of the operation depends on the operating conditions of temperature, pressure, residence time in the separation vessel, but it mainly depends on the nature of the precipitation solvent and on the solvent-to-charge ratio. For efficiency, this operation involves the use of a large volume of solvent since the ratio by volume of the solvent to the charge generally ranges from 2/1 to 15/1 and mostly from 3/1 to 8/1.
The more commonly used apparatuses for this operation consist of:
either an extractor-settler from the bottom of which are recovered the coagulated asphalts together with a small fraction, generally from about 5 to 15%, of the solvent.
From the top of the extractor-settler is recovered a mixture of hydrocarbon oil, free of asphalts, forming the so-called "deasphalted oil", together with the major part--usually about 85-95%--of the solvent used in this operation.
or in a multistage column: the charge is introduced into the upper half part of the column, the solvent being fed counter-currently to the column bottom. Asphalt is recovered from the bottom and the mixture of deasphalted oil and solvent is recovered from the heated top.
According to the nature of the solvent, this mixture is recovered at a temperature usually ranging from 60.degree. to 220.degree. C., under such a pressure that the solvent and the deasphalted oil mixture remain in liquid state. The deasphalted oil content of this mixture, extracted from the top, obviously depends on the nature of the charge and on the amount of solvent used in proportion to the charge as well as on the operating conditions. Generally, the proportion by weight of deasphalted oil in this top fraction is from 10 to 40% and mostly from 20 to 38%.
The treatment of this fraction consisting of separating the solvent from the deasphalted oil is theoretically simple in view of the respective different volatilities of the constituents. As a matter of fact, the deasphalted oil boils under normal pressure within a temperature range far aove 350.degree. C.; however the solvent evaporation requires a considerable power expense in view of the large amount of solvent used.
Accordingly, many processes for solvent evaporation tending to decrease the power expense have been disclosed in the prior art, such for example as in U.S. Pat. No. 2,943,050 disclosing a process using successively two flash eveporators, French Pat. No. 2,425,472 disclosing a process using three continuous flash distillation zones operating under constant temperature and pressure conditions, French Pat. No. 2,490,103 claiming a process for recovering solvent in several steps, in falling-film evaporators providing also for a nucleate boiling. Some of these processes provide for about 50% reduction of the power expense of the operation, but nevertheless the corresponding cost remains high.
Another process for reducing the power cost consists of heating the deasphalted oil and solvent mixture to a temperature higher than the critical temperature of the solvent. Under these supercritical conditions, the solubility of the deasphalted oil in the solvent decreases and a separation of the two phases occurs. This technique has been disclosed in many patents and paper, such as U.S. Pat. Nos. 2,940,920, 4,239,616, 4,290,880 and 4,305,814.
However, although it is true that this process provides for substantial power saving during the step of recovering the deasphalting solvent, it suffers from the disadvantage of requiring higher temperature and pressure conditions than those used in the conventional solvent recovery and of requiring an extensive thermal exchange between the deasphalted oil-deasphalting solvent mixture and the separated deasphalting solvent.
The purpose of the present invention is to provide a new deasphalting process comprising a step of separating the deasphalted oil from the deasphalting solvent by ultrafiltration, in the liquid phase, without change of state, this separation being conducted at high temperature and resulting in substantial power saving as compared with the prior art processes.
The use of organic semipermeable membranes in processes for separating various compounds is well known; such processes are usually called "reverse osmosis" or "ultrafiltration" processes. These membranes usually consist of polymer materials such as cellulose esters, regenerated cellulose, polyamides, polyvinyl chloride or cross-linked polyethylene, polyacrylonitrile and polysulfone.
Their use in petrochemistry is considerably limited by their poor resistance to hydrocarbon solvents and their very low thermal stability.
The French Pat. No. 2,482,975 illustrates for prior art ultrafiltration processes using inorganic membranes for separating hydrocarbon products in the liquid state at a temperature higher than 80.degree. C. This patent states the use of inorganic ultrafiltration barriers coated with a sensitive layer of at least one metal oxide having a permeametry radius ranging from 50 to 250 .ANG.; it is adapted to the regeneration of used oils by removing their impurities which are retained by the barriers and it may also be used to reduce the asphaltene content of the hydrocarbon charges. For this latter application, the process appears to be unsatisfactory since the rate of removal of asphaltenes is still low, as shown in example 2 of said French patent.